Method of implanting a unicondylar knee prosthesis

ABSTRACT

A method of implanting a lateral unicondylar knee prosthesis in a lateral articulating portion of a femur having a lateral condyle for engagement with a tibia, the method including the steps of: bending a patients knee such that the knee is not in full extension; making an incision through the skin, muscle, and other soft tissue until the damaged bone surfaces are exposed; resecting an end portion of the lateral tibia; resecting an end portion of the lateral femoral condyle; attaching a tibial prosthetic component to the resected end portion of the tibia; attaching a lateral femoral condyle prosthetic component to the resected end portion of the lateral femoral condyle; determining the thickness of a mobile bearing member with the knee in full extension; and inserting the mobile bearing member between the tibial prosthetic component and the femoral prosthetic component.

FIELD OF THE INVENTION

The present invention relates to a method for knee surgery, and inparticular, to a method of implanting a unicondylar knee prosthesis.

BACKGROUND OF THE INVENTION

The knee joint is where the end of the upper leg bone (femur) meets thetop of the lower leg bone (tibia). The end of the femur consists of twocondyles, which are like runners or wheels on each side of the bone thatboth roll and slide with respect to the other surface. These condylesare located on top of the tibia, which is like a platform that isslightly dished on the medial side but slightly convex on the lateralside. Thus, on each side of the joint, there is an area of contactbetween the two bones. When the knee is bent, the condyles of the femurroll and slide on top of the tibia at these two areas of contact. Athird bone, the kneecap (patella), glides over the front and end of thefemur.

The two condyles form two articular bodies of the femur and arerespectively named its lateral (outer) and medial (towards the middle)condyles. These diverge slightly distally (front) and posteriorly(rear), with the lateral condyle being wider in front than at the backwhile the medial condyle is of more constant width. The radius of thecondyles' curvature in the sagittal plane becomes smaller toward theback. This diminishing radius produces a series of involute midpoints(i.e. located on a spiral). The resulting series of transverse axes(along the involute midpoints) permit the sliding and rolling motion inthe flexing knee while ensuring the collateral ligaments aresufficiently lax to permit the rotation associated with the curvature ofthe medial condyle about a vertical axis.

Motion of a natural knee is kinematically complex. During a relativelybroad range of flexion and extension, the articular or bearing surfacesof a natural knee experience rotation, medial and lateral angulation,translation in the sagittal plane, rollback and sliding. The knee joint,in combination with ligaments and muscles, attempt to produce thiscomplex knee motion, as well as absorb and control forces generatedduring the range of flexion.

In a healthy knee joint, the surfaces of these bones are very smooth andcovered with a tough protective tissue called cartilage. Arthritiscauses damage to one or more of the bone surfaces and cartilage wherethe bones rub together. In particular, osteoarthritis or “wear-and-tear”arthritis is a condition where the surface of the joint is damaged andthe surrounding bone grows thicker, as a result of bone against bonefriction. This friction can cause severe pain and eventually loss ofmovement.

One option for treating the permanent deterioration of the cartilage istotal knee replacement surgery. In a total knee replacement procedurethe end surfaces of the tibia and femur, and the posterior surface ofthe patella, are resurfaced. Generally, the surface that covers thefemoral section is made from smooth metal or ceramic, while the surfaceattached to the tibia is constructed of metal and a high-densitypolyethylene. The patella is also resurfaced with a high-densitypolyethylene.

While known knee joint prostheses have proven to be effective inreplacing the anatomical knee joint, they nevertheless have severaldisadvantages. For example, knee joint prostheses sometimes lackadaptability to implant conveniently with a given patient. In thisregard, in a normally shaped femur, the central canal is typicallyoffset from the centre of the femoral articulating surfaces.Furthermore, the central femoral canal may present various valgus anglesfrom one patient to another.

In some cases, there may be significant damage on only one side of thejoint or to only one of the condyles. In these cases, a partial(unicompartmental) knee replacement may be considered. In a partial kneereplacement, only one side of the joint is resurfaced.

Accordingly, it is desirable to develop a method for knee surgery, andin particular, a method of implanting a unicondylar knee prosthesiswhich may help make surgery, recovery, and rehabilitation faster andeasier for the patient and that seeks to preferably mitigate, alleviateor eliminate one or more of the disadvantages mentioned above singly orin any combination.

Any discussion of documents, acts or knowledge in this specification isincluded to explain the context of the invention. It should not be takenas an admission that any of the material forms part of the prior artbase or the common general knowledge in the relevant art.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides a method ofimplanting a lateral unicondylar knee prosthesis in a lateralarticulating portion of a femur having a lateral condyle for engagementwith a tibia, the method comprising the steps of: bending a patientsknee such that the knee is not in full extension; making an incisionthrough the skin, muscle, and other soft tissue until the damaged bonesurfaces are exposed; resecting an end portion of the lateral tibia;resecting an end portion of the lateral femoral condyle; attaching atibial prosthetic component to the resected end portion of the tibia;attaching a lateral femoral condyle prosthetic component to the resectedend portion of the lateral femoral condyle; determining the thickness ofa mobile bearing member with the knee in full extension; and insertingthe mobile bearing member between the tibial prosthetic component andthe femoral prosthetic component.

The position of the lateral femoral condyle may be determinedanatomically with reference to the femur. The step of resecting the endportion of the tibia may comprise sawing vertically a cut through thecentre of a patella tendon positioned in line with the medial side ofthe lateral femoral condyle; and sawing horizontally a cut that removesbone to accommodate the tibial prosthetic component and the mobilebearing member with the knee in full extension. Sawing horizontally mayfurther comprise removing bone to approximately 8 mm below the originaltibial articular surface or 2 to 3 mm below the eburnated bone of thetibia. The tibial prosthetic component may include a smooth articularsurface and a bone-contacting surface opposite the articular surface.The smooth articular surface may be formed in a convex shape. Thebone-contacting surface opposite the articular surface may furtherinclude a projection extending from the bone-contacting surface. Thestep of attaching the tibial prosthetic component may include resectingthe bone to accommodate the projection extending from thebone-contacting surface; and cementing the tibial prosthetic componentto the tibia. The step of attaching the tibial prosthetic component mayinclude resecting the bone to accommodate the projection extending fromthe bone-contacting surface; and attaching a cementless tibialprosthetic component to the tibia. The step of resecting the end portionof the lateral femoral condyle may include drilling a hole in a notchbetween the lateral femoral condyle and the medial femoral condyle toaccommodate an intramedullary rod; inserting a lateral femoral drillguide; adjusting the leg and the lateral femoral drill guide to beparallel to the intramedullary rod; drilling two holes through thelateral femoral drill guide; attaching a posterior saw guide to thelateral femoral compartment; and milling the bone from the lateralfemoral condyle in both anterior and posterior directions. The femoralprosthetic component may include a smooth articular surface and abone-contacting surface opposite the articular surface. The smootharticular surface may be formed in a convex shape. The bone-contactingsurface opposite the articular surface may further include a spigotextending from the bone-contacting surface. The step of attaching thelateral femoral prosthetic component may include cementing the lateralfemoral prosthetic component to the femur. The step of attaching thelateral femoral prosthetic component may include attaching a cementlesslateral femoral prosthetic component to the femur. The mobile bearingmember may be a biconcave bearing having a first bearing surface thatarticulates with the smooth articular surface of the tibial prostheticcomponent and a second bearing surface that articulates with the smootharticular surface of the femoral prosthetic component. The tibialprosthetic component, the femoral prosthetic component and the mobilebearing member may be made of a material selected from the group oftitanium, titanium alloy, cobalt chrome alloy, ceramic, biocompatiblecomposite, polymer, niobium, and steel alloy.

According to a further aspect, the present invention provides a methodof implanting a lateral unicondylar knee prosthesis in a lateralarticulating portion of a femur having a lateral condyle for engagementwith a tibia, the method including the steps of: bending a patients kneesuch that the knee is not in full extension; making an incision throughthe skin, muscle, and other soft tissue until the damaged bone surfacesare exposed; resecting an end portion of the lateral tibia; sizing atibial template component by fully extending the knee; resecting an endportion of the lateral femoral condyle; sizing a lateral femoraltemplate component by fully extending the knee; and sizing a mobilebearing template member for insertion between the tibial templatecomponent and the lateral femoral template component.

The position of the lateral femoral condyle may be determinedanatomically with reference to the femur. The step of resecting the endportion of the tibia may include sawing vertically a cut through thecentre of a patella tendon positioned in line with the medial side ofthe lateral femoral condyle; and sawing horizontally a cut that removesbone to accommodate the tibial prosthetic component and the mobilebearing member with the knee in full extension. Sawing horizontally mayfurther include removing approximately 8 mm below the original tibialarticular surface or 2 to 3 mm below the eburnated bone of the tibia.The tibial template component may include a smooth articular surface anda bone-contacting surface opposite the articular surface. The smootharticular surface may be formed in a convex shape. The step of resectingthe end portion of the lateral femoral condyle may include drilling ahole in a notch between the lateral femoral condyle and the medialfemoral condyle to accommodate an intramedullary rod; inserting alateral femoral drill guide; adjusting the leg and the lateral femoraldrill guide to be parallel to the intramedullary rod; drilling two holesthrough the lateral femoral drill guide; attaching a posterior saw guideto the lateral femoral compartment; and milling the bone from thelateral femoral condyle in both anterior and posterior directions. Thelateral femoral template component may include a smooth articularsurface and a bone-contacting surface opposite the articular surface.The smooth articular surface may be formed in a convex shape. Thebone-contacting surface opposite the articular surface further mayinclude a spigot extending from the bone-contacting surface. The methodmay further include replacing the tibial template component by insertinga tibial prosthetic component; replacing the lateral femoral templatecomponent by inserting a lateral femoral condyle prosthetic component;and replacing the mobile bearing template member by inserting a mobilebearing member. A final assessment of the implant of the lateralunicondylar knee prosthesis may be performed and the tibial prostheticcomponent and the femoral prosthetic component are cemented into theirrespective bones. A final assessment of the implant of the lateralunicondylar knee prosthesis may be performed and the tibial prostheticcomponent and the femoral prosthetic component are attached to theirrespective bones using cementless technology. The mobile bearing membermay be a biconcave bearing having a first bearing surface thatarticulates with the smooth articular surface of the tibial prostheticcomponent and a second bearing surface that articulates with the smootharticular surface of the femoral prosthetic component. The tibialprosthetic component and the tibial prosthetic template component, thefemoral prosthetic component and the femoral prosthetic templatecomponent, and the mobile bearing member and the mobile bearing templatemember may be made of a material selected from the group of titanium,titanium alloy, cobalt chrome alloy, ceramic, biocompatible composite,polymer, niobium, and steel alloy.

According to a still further aspect, the present invention provides akit for a lateral unicondylar knee prosthesis housed in a sterilisationcase, the kit including: a set of femoral components including: at leastone femoral drill guide; at least one femoral prosthetic component; atleast one femoral prosthetic template component; a femoral instrumentset; a set of tibial components including: at least one tibialprosthetic component; at least one tibial prosthetic template component;a tibial impactor; a tibial instrument set; a set of bearing componentsincluding: at least one mobile bearing member; at least one mobilebearing template member; a bearing insert/removal tool; a set of feelergauges for determining correct sizing of components; and a set ofinstructions for performing a lateral unicondylar knee implant surgery.

The tibial prosthetic component and the femoral prosthetic component maybe cemented into their respective bones. The tibial prosthetic componentand the femoral prosthetic component may be attached to their respectivebones using cementless technology. The tibial prosthetic component andthe tibial prosthetic template component, the femoral prostheticcomponent and the femoral prosthetic template component, and the mobilebearing member and the mobile bearing template member may be made of amaterial selected from the group of titanium, titanium alloy, cobaltchrome alloy, ceramic, biocompatible composite, polymer, niobium, andsteel alloy.

The present invention provides a partial knee replacement procedure thatnot only helps the knee joint function better but has also proven toimprove both recovery and rehabilitation after surgery. The procedureinvolves removing only the diseased portion of the knee, traditionaltotal knee replacement surgery involves removing or resurfacing moreparts of the knee, including both condyles and often the underside ofthe kneecap.

The present invention is a less invasive approach to knee surgery and isparticularly suited to patients who are still in the early stages ofosteoarthritis. This means only the parts (the medial, ‘inner’compartment or lateral ‘outer’ compartment) of the knee that have beendamaged by the disease need replacing, therefore avoiding having toundergo a total knee replacement. This procedure preserves the healthyparts of the joint, as it only replaces one side of the knee, leading toan artificial joint with a function that is closer to the natural kneesmovements.

The component parts of the prosthesis have been designed to havesurfaces that fit one another in all positions such that thekinematically complex movements of the knee are retained. This alsomeans that component parts of the prosthesis wear out very slowly. Afurther benefit of this procedure is that by retaining all the undamagedparts, in particular the cruciates and ligaments, the joint bends betterand has a more natural function.

These and other objects, along with the advantages and features of thepresent invention herein disclosed, will become apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and canexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. In addition, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a perspective view of the knee joint, without the muscle andconnective tissue, showing a lateral condyle and a medial condyle of thefemur in accordance with the present teachings, and also showingreference planes with respect to the knee joint;

FIG. 2 a is a perspective view of the knee joint showing a healthy kneejoint;

FIG. 2 b is a perspective view of a knee joint showing progressive kneejoint degeneration that may be due to osteoarthritis;

FIG. 3 is a perspective view of a knee showing a femoral componentsecured to the lateral condyle and a tibial component implanted over thelateral articular surface of the tibia and a mobile bearing inaccordance with the present invention;

FIG. 4 is a schematic view of a tibial component in accordance with thepresent invention;

FIG. 5 is a schematic view of a lateral femoral condylar component inaccordance with the present invention;

FIG. 6 is a schematic view of a biconcave bearing member in accordancewith the present invention;

FIG. 7 is a perspective view of a knee in full extension with a tibialtemplate in place for assessing if enough bone has been resected fromthe tibia;

FIG. 8 is a perspective view of a knee with a line drawn down the centreof the lateral femoral condyle;

FIG. 9 is a perspective view of the knee with the femoral drill guide inplace for drilling holes in the femur in accordance with the presentinvention;

FIG. 10 a is a perspective view of the knee with the posterior saw guidemounted on the lateral femoral condyle;

FIG. 10 b is a perspective view of the knee with a ‘0’ spigot in placefor milling the femur;

FIG. 10 c is a perspective view of the knee in full extension with bothfemoral and tibial template components in place either side of a feelergauge for assessing the flexion gap;

FIG. 11 a shows a perspective view of the knee with a trial bearing inplace between the femoral component and the tibial component;

FIG. 11 b shows a perspective view with the trial components and a trialbearing in place with the knee in full extension for a final assessment;

FIG. 12 is a flowchart illustrating the method in accordance with anembodiment of the present invention; and

FIG. 13 shows a detailed flow chart illustrating the method inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

The present invention will be discussed hereinafter in detail in termsof various embodiments of a method of implanting a unicondylar kneeprosthesis. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be obvious, however, to those skilled in the art thatthe present invention may be practiced without these specific details.

With reference to FIG. 1, it will be appreciated that theanterior-posterior plane 100 and the medial-lateral plane 200 are notexactly and specifically located on the body but can provide generalguidance as to orientation and location. As such, alignment of theprosthetic 20 (FIG. 3) to the anterior-posterior plane 100 and thespigot (not shown) of the femoral component 30 and the femur 15 to themedial-lateral plane 200 can provide a general orientation of thefemoral component 30 relative to the femur 15 and the tibia 16. Alsoshown in FIG. 1 is the line 13 that defines a longitudinal axis of thetibia 16.

With reference to FIG. 1, the incision 11 (or multiple incisions) can bemade at various locations around the knee joint 1 and can aid ininsertion of the femoral component 30 and/or the tibial component 40.While a minimally invasive incision can be used, the femoral component30 and/or the tibial component 40 can be compatible with other incisionsand/or other suitable medical equipment.

FIG. 2 a shows a healthy knee joint 1 and FIG. 2 b shows a knee jointshowing progressive knee joint degeneration. The knee joint 1 is wherethe end of the femur 15 meets the top of the tibia 16. The end of thefemur 15 consists of two condyles 60, 70. These condyles 60, 70 sit ontop of the tibia 16, which is like a platform that is slightly dished oneach side. Thus, on each side of the joint 1, there is an area ofcontact between the two bones. When the knee is bent, the condyles 60,70 of the femur 15 roll and slide on top of the tibia 16 at these twoareas of contact. A third bone, the kneecap (patella) 80, glides overthe front and end of the femur 15.

In a healthy knee joint (FIG. 2 a), the surfaces of these bones are verysmooth and covered with cartilage—a tough protective tissue. Thearticular disks of the knee-joint are called menisci 91, 92 because theyonly partly divide the joint space. These two disks, the medial meniscus91 and the lateral meniscus 92, consist of connective tissue withextensive collagen fibers containing cartilage-like cells. Strong fibersrun along the menisci 91, 92 from one attachment to the other, whileweaker radial fibers are interlaced with the former. The menisci 91, 92are flattened at the centre of the knee joint, fused with the synovialmembrane laterally, and can move over the tibial surface 16. Also in ahealthy knee joint is the articular cartilage 90, also called hyalinecartilage, which is the smooth, glistening white tissue that covers thesurface of all the diarthrodial joints in the human body.

The menisci 91, 92 serve to protect the ends of the bones from rubbingon each other and to effectively deepen the tibial sockets into whichthe femur 15 attaches.

They also play a role in shock absorption, and may be cracked, or torn,when the knee is forcefully rotated and/or bent. FIG. 2 b shows a kneejoint 1 which arthritis has caused damage to one or more of the bonesurfaces and cartilage where the bones rub together. In particular,osteoarthritis is a condition where the surface of the joint is damagedand the surrounding bone grows thicker, resulting in bone against bonefriction. This friction can cause severe pain and eventually loss ofmovement. In some cases, there may be significant damage on only oneside of the joint 1 or to only one of the condyles 60, 70. In thesecases, a partial (unicompartmental) knee replacement may be considered.In a partial knee replacement, only one side of the joint 1 isresurfaced.

Up until now, surgeons have used the same techniques to replace thelateral condyle 60 as they have used to replace the medial condyle 70.It has not previously been appreciated that better results are obtainedby using a different operative procedure for the lateral condyle 60. Bybetter results, we mean fewer problems post-operative, less pain andrecovery and rehabilitation is faster and easier for the patient,reduction of bearing dislocation, improved range of movement andimproved kinematics of the knee.

It has now been appreciated that the different forces and motionexperienced in use by medial and lateral condylar 60, 70 replacementinserts can be better achieved by fitting the lateral condylar 60replacement differently from how the medial condylar 70 replacement isfitted. In particular, sizing the lateral condylar 60 replacement insertwith the leg fully extended (straight) is better, despite not being whatis done for medial condylar 60 replacement. It is somewhatcounterintuitive to size the insert whilst the knee 1 is straight—theexterior ligament is slack in this position, and sizing the insert witha slack ligament may instinctively feel wrong, but it is better.

The present invention has been developed for the above case where onlyone side of the joint 1 or one of the condyles 60, 70 has been damaged.The method for knee surgery, and in particular, a method of implanting alateral unicondylar knee prosthesis 20 has been developed to makesurgery, recovery, and rehabilitation faster and easier for the patient.

The procedure and reasoning for a lateral unicondylar partial kneeimplant will now be described with reference to FIG. 3. FIG. 3 shows aperspective view of a knee 1 showing a femoral component 30 secured tothe lateral condyle 60 and a tibial component 40 implanted over thelateral articular surface of the tibia 16. A mobile bearing member 50 isinserted between the tibial prosthetic component 40 and the femoralprosthetic component 30. Further detail of the tibial component 40 willbe described below with reference to FIG. 4, the femoral prostheticcomponent 30 will be described with reference to FIG. 5 and the mobilebearing member 50 will be described with reference to FIG. 6.

There are many differences between the medial and lateral compartmentsof the knee 1. One of the most important is that in flexion, the lateralcollateral ligament (LCL) is slack whereas the medial collateralligament (MCL) is tight. Therefore, it is not possible to determine theposition of the lateral femoral condyle 60 from ligament balance, as isdone in operations to replace the medial compartment (discussed in moredetail latter). Also, isolated lateral arthritis is a disease of flexionand tends to be maximal at 40° flexion. It is therefore different frommedial osteoarthritis, which is a disease of extension.

Instead, an aim of one aspect of the present invention is to positionthe component 20 anatomically. This is possible because in lateralosteoarthritis there is usually little damage to the femur 15 at 90°flexion (or thereabouts) and in full extension (or thereabouts), so thefemoral component 30 can be positioned with reference to these surfaces.Once the tibial 40 and femoral 30 components are implanted, theappropriate mobile bearing 50 is selected in full extension, when theLCL is tight, thus restoring leg alignment to the predisease situation.Selecting the mobile bearing 50 with the knee in extension, rather thanflexed, as with a medial unicondylar replacement, is another difference.

FIG. 4 shows the tibial component 40 used to replace a singlecondylar-bearing portion of a human tibia 16. The tibial component 40includes a body portion 2 having a substantially flat bone-engagingregion 4 that has a raised border disposed around its periphery defininga bone engaging surface 6. The body portion 2 provides a domed articularsurface 8 having an abutment 10 projecting therefrom. In use the domedarticular surface 8 supports a meniscal or mobile bearing component 50for engagement with a prosthetic femoral component 30 in such a manneras to enable the prosthesis to serve as a replacement for the naturaltibial articular surface and to restore substantially normal anatomicalmovement of the femur 15 and tibia 16. As is well known in the art theabutment 10 aids to prevent dislocation of the mobile bearing component50 towards the cruciate ligaments.

The body portion 2 is fixed to the tibia by means of an elongate flangeor keel 14 that is anchored directly in the tibia 16. The flange 14 isconnected to the bone-engaging region 4 of the body portion 2. In orderto implant the prosthesis, a hole of a diameter substantially equal tothe width w₂ of the flange 14, is drilled into the already resectedportion of the tibia 16.

The end of the flange 14 is inserted into the hole and the tibialcomponent 40 prosthesis is driven across the flat surface formed on thetibia 16 until the entire flange 14 is anchored in the tibia 16. Priorto insertion of the flange 14 into the tibia 16, bone cement may beapplied to the bone-engaging region 4.

The femoral prosthetic component 30, the tibial prosthetic component 40and the mobile bearing member 50 are typically constructed ofbiomaterials that are compatible with use in the body. Some examples ofbiomaterials include titanium, titanium alloy, cobalt-chrome, cobaltchrome alloy, ceramics, biocompatible composites, polymers includingpolyethylene, steel alloys, niobium and Trabecular Metal™ material.Niobium is typically used in patients who are metal sensitive. Thepreferred material for the mobile bearing component 50 is high-densitypolyethylene, which provides the articular surface with the appropriatelubricity characteristics for the proper functioning of the kneeprosthesis 20.

FIG. 5 shows a lateral femoral condylar component 30 having a spigot 39and lateral femoral engaging surfaces 43 to engage the lateral femoralcondyle 60. The body portion provides a domed articular surface 44. Inuse the domed articular surface 44 supports a meniscal or mobile bearingcomponent 50 for engagement with a prosthetic tibial component 40 insuch a manner as to enable the prosthesis to serve as a replacement forthe natural femoral articular surface and to restore substantiallynormal anatomical movement of the femur 15 and tibia 16.

As described above the LCL is slack in flexion therefore the lateralcompartment can readily and easily be pulled apart or distracted.Bending the leg so as to make the LCL slack is a part of manyembodiments of the invention. Therefore the surgery can be carried outwith the leg/knee at any angle provided the leg is not fully extended.That means in some embodiments at any angle within the range ofapproximately 10 degrees to 90 degrees (or perhaps more) flexion. Insome embodiments a preferred range of knee bend is between 20 degreesand about 90 degrees (say 90 degrees plus or minus 5 or 10 degrees), orbetween 30, or 40, or 50, or 60, or 70, or 80 degrees, and about 90degrees. Bending the knee whilst fitting the unicondylar lateral implantcauses a higher risk of dislocation of the bearing implant 50 laterallythan medially. In order to prevent this dislocation, in many embodimentsa biconcave bearing 50 (FIG. 6) is implanted between the femoralcomponent 30 and the tibial component 40 which enhances entrapment andtherefore aids in the prevention of dislocation.

A biconcave bearing 50 is essentially required due to the lateral tibialplateau (articulating surface) being convex or more of a D-shape thanthe medial plateau. This is mirrored in the shape of the components usedfor the bearing 50. As shown in FIG. 6, the top and bottom surfaces 52,53 of the mobile bearing 50 are concave shaped to mirror the shapes ofthe lateral tibial plateau and the lateral femoral condyle 60. The twoconcave surfaces 52, 53 or outer faces are curved inward and are joinedby the surface 54 surrounding the bearing 50.

Although the lateral femoral condyle 60 is more circular in the sagittalplane than the medial, it is less circular in cross section, which canbe disorientating. The patella 80 and patella tendon lie in front of thelateral femoral condyle 60, which also makes access difficult. Anotherconsideration, which should be observed, is the popliteus (a thin, flat,triangular muscle at back of the knee, the action of which assists inbending the knee and in rotating the leg toward the body) may bowstringacross the back of the lateral compartment and cause mobile bearing 50dislocation.

During flexion and extension, there is a large amount of movement of thelateral femoral condyle 60 on the tibia 16. In high flexion the mobilebearing 50 overhangs the back of the tibial component 40. Therefore, theback of the knee 1 has to be free of obstructions. As the knee 1extends, as well as moving forwards the mobile bearing 50 moves mediallyso the tibial component 40 has to be markedly internally rotated.

The procedure for implanting a prosthetic knee 20 to the lateral femoralcondyle 60 is broadly similar to that of implanting a prosthetic knee 20to the medial femoral condyle 70 and will be described with reference toFIGS. 7 to 13. An incision 11 is made over the junction between thecentral and lateral third of the patella 80. It starts about 2 cm abovethe patella 80 and ends just below and lateral to the tibial tuberosity.The retinaculum is incised lateral to the patella 80 and the ligamentumpatellae. The retinacular incision is extended upwards into thequadriceps tendon to allow the patella 80 to sublux medially.

As described above the patella 80 and patella tendon are in front of thelateral femoral condyle 60 making access to the lateral compartmentdifficult. Occasionally it is necessary to remove a strip of bone 5 to 7mm wide from the lateral side of the patella 80 to gain adequateexposure. Generous excision of the fat pad, particularly in theintercondylar notch 33, is necessary to gain adequate access.

The next step in the procedure is to inspect the joint 1 and assess thestate of the anterior cruciate ligament, the patello femoral joint andthe medial compartment. If any of these are in a poor state, a totalknee replacement would be required rather than a unicondylarreplacement. Osteophytes (a small, abnormal bony outgrowth) are thenremoved along with the removal of the lateral notch osteophytes and careis taken to define the medial border of the lateral femoral condyle 60.

The next step is to resect the tibial plateau. Starting with the tibia16, the patella tendon is exposed and its lower portion is splitcentrally, for example with a knife. A vertical saw cut is made throughthe patella tendon with the knee flexed to 90°. The saw blade shouldtouch the medial side of the lateral condyle 60. The two landmarks, thecentre of the tendon and the medial side of the lateral condyle 60define the position of the cut and ensure it is adequately internallyrotated. The saw cut will usually be at the correct depth when its upperedge is level with the bone surface.

The lateral tibial resection guide is applied in a manner similar tothat used on the medial side. For example, the lateral tibial resectionguide would be strapped around the ankle and adjusted to be parallel tothe tibial crest. It should be positioned parallel to the vertical sawcut. The aim is for the horizontal is to remove sufficient bone toaccommodate the tibial prosthetic component 40 and the mobile bearingmember 50 with the knee in full extension. This is achieved by making ahorizontal saw cut to a depth of approximately 8 mm below the originaltibial articular surface. If the site of the original surface can not beestimated, the cut should be approximately 2 or 3 mm below the eburnatedbone. As the domed tibial component is thicker than the flat medialcomponent, the lateral tibial resection should be thicker than themedial.

The horizontal saw cut should be made with care so as to avoid damage tothe soft tissues laterally, in particular the iliotibial tract and theposteriorly positioned lateral collateral ligament. The tibial plateauis removed and is then sized to the appropriate template 41. To confirmthat enough bone has been resected from the tibia 16, the tibialtemplate 41 should be inserted and the knee fully extended as shown inFIG. 7. If a No. 4 feeler gauge (or No. 3 in small women) 42 cannot beinserted in full extension the tibia 16 should be recut. Another of thedifferences between the operation to replace the medial compartmentcompared with the new operation to replace the lateral compartment isthat to confirm enough bone has been resected from the tibia 16 in thelateral compartment the assessment is made in full extension, not 20°flexion as is the case on the medial side.

Once the tibia 16 has been resected and enough bone has been removedfrom the tibial plateau, the next step is to resect the femoral lateralcondyle 60 to allow for the insertion of the femoral component 30. Theaim of femoral preparation is to place the femoral component 30 as closeto the anatomical position as possible. The size of the lateral femoralcomponent 30 is selected using the same principles as is the medialcomponent, i.e. is selected to be a similar size to that of the naturalcondyle.

FIG. 8 shows a line 31 drawn down the centre of the lateral femoralcondyle 60. An intramedullary rod (IM) (not shown) is inserted into arecess 32 about a centimeter anterior and a half centimeter lateral tothe most antero-lateral corner of the notch 33. The rod is left in placethroughout the operation except when gaps are being measured. Care istaken not to damage the extension mechanism when flexing the knee 1; thepatella 80 should be guided around the rod.

FIG. 9 shows the tibial template 41, appropriate feeler gauge 42 andlateral/femoral drill guide 45, inserted into the gap between thelateral femoral condyle 60 and the articulating surface of the tibia 16.Alignment of the femoral drill guide 45 is fundamentally different tothe medial side and can be disorientating due to the lateral condylebeing less circular in cross section but more circular in the sagittalplane. The extramedullary alignment rod can therefore be used as anadditional guide.

The position of the leg and guide are adjusted so that the IM rod isparallel to the drill guide 45 in both coronal and sagittal planes. Theextramedullary rod should point to the femoral head and be parallel tothe femur 15 in the sagittal plane. Achieving correct alignment is oftendifficult but 10° mal-alignment is acceptable. The mediolateral positionof the drill guide 45 should be adjusted until it is central or justlateral to the centre of the lateral femoral condyle 60. This is bestchecked by seeing the line 31 drawn down the lateral femoral condyle 60within the 6 mm drill hole 35 of the drill guide 45. The feeler gauge 42will not be parallel to the wall of the tibial template 41 and may notbe touching the tibial template 41.

To prevent antero-lateral overhang of the femoral component 30 the drillguide 45 often needs to be positioned in slight internal rotation. Usingthe drill guide 45 two drill holes 34, 35 are made into the lateralfemoral condyle 60. The drill guide 45 is then replaced by the posteriorsaw guide 36 as shown in FIG. 10 a. The posterior saw guide 36 is usedin the same manner as in the medial compartment to guide the saw cutsuch that the quantity of bone and cartilage removed equals thethickness of the posterior part of the implant. Any remnants of theposterior horn (at the back of the knee) of the meniscus are excised butthe popliteus is not divided. It is the posterior horn of the meniscusthat is more commonly injured.

In flexion, the lateral ligaments are lax and the lateral structures areextensible, so the flexion gap cannot be measured accurately. Therefore,the femoral component 30 position cannot be determined by ligamentbalance. Instead, the aim is to position the femoral component 30anatomically. This is achieved by milling the same thickness ofcartilage and bone from the inferior surface of the femur to match thedistal thickness of the implant. This is usually achieved by millingwith a ‘4’ spigot. Primary milling is undertaken with the ‘0’ spigot 37as shown in FIG. 10 b. Trial components 38, 41 are inserted and theextension gap is measured with the knee fully extended using a feelergauge 42 as shown in FIG. 10 c. The tibial component is inserted thenthe knee extended.

Prior to using a ‘4’ spigot and milling 4 mm from the distal femur 15 anassessment is made as to what effect this will have on the flexion gap.This is done by inserting a feeler gauge 42, 4 mm thicker than themeasured extension gap. If this feeler gauge 42 is just gripped with theleg hanging dependant in flexion then milling with a ‘4’ spigot shouldproceed. If the feeler gauge 42 is very tight, as may occur if there isa deep distal defect, then less bone should be removed. If the feelergauge 42 is grossly loose then a ‘5’ spigot may be used. In the vastmajority of situations, a spigot higher than ‘5’ should not be used asthis will elevate the tibial joint line, and increase the risk ofdislocation. Following secondary milling, trial components 38, 41 areinserted and an assessment of the gaps is made. It is likely that theflexion gap will be looser than the extension gap. If a varus load isapplied, the flexion gap may open considerably. This is a manifestationof the loose lateral collateral ligament in flexion and is to beexpected. Finally, with regard to the femur 15, bone is removedanteriorly and posteriorly on the femur 15 to prevent impingement.

FIGS. 11 a and 11 b show a trial template bearing 51 inserted in betweenthe femoral template 38 and the tibial template 41. The trial templatebearing 51 thickness is selected in full extension. In this position,the trial template bearing 51 should be slightly lax. In flexion, theknee 1 may be very lax. This should be accepted. The medio-lateralposition of the trial template bearing 51 should be assessed. If thetrial template bearing 51 hits the wall, which is most likely to occurin full extension, the vertical tibial cut should be redone 1 to 2 mmmore medial.

As described above, implant systems are affixed to the body by eithercementing the prosthesis with bone cement or cementless such as biologicingrowth or a combination of both of these. When using bone cement aspecial type of acrylic bone cement may be used to secure some or all ofthe implant components to the bone. However if using the cementlessoption the prosthesis is implanted into the bone without cement. Thesecomponents have a special porous coating that allows tissue to grow intoit for fixation. If using a combination of the two a surgeon may choosea combination of cement and cementless attachment, depending upon theimplant components and the condition of the bone around the knee joint.

Once the final components 30, 40 and 50 have been chosen, a finalassessment should be carried out prior to fixing the tibial component 40and the femoral component 30 to their respective resected bone regions.Once final assessment is completed, care should be taken to ensure thatthere is nothing that might displace the mobile bearing 50 and cause adislocation. If popliteus prevents the mobile bearing 50 from movingposteriorly it should be divided.

The tibial component 40 as described in FIG. 4 above has a projection orkeel 14 for which further bone needs to be resected from the tibialplateau to accommodate the keel 14 of the tibial component 40. Oncepreparation of the surfaces with multiple drill holes has been completedthe components 30, 40 are cemented in and excess cement is removed. Atrial reduction is undertaken to determine the appropriate mobilebearing 50 thickness and to ensure there are no problems with thereconstruction. An appropriate mobile bearing 50 is one that is nottight in full extension. The definitive mobile bearing 50 is theninserted. This can usually be inserted from the front. If this is notpossible consider using a thinner bearing. On the lateral side it isalways better to leave the knee lax. If it is impossible to insert themobile bearing 50 from the front, it can be inserted obliquely from anantero-lateral direction.

The knee must not be over stuffed resulting in the bearing being tootight. This will lead to pain and increase the risk of dislocation. Ithas been found that the best results are achieved in patients that havesome residual valgus.

Orthopedic surgery or orthopedics (also spelled orthopaedics) is thebranch of surgery concerned with conditions involving themusculoskeletal system. In order to provide the orthopedic surgeon withthe tools and equipment required to perform a knee implant inparticular, a lateral unicondylar knee implant a set or collection oftools, supplies, and instructional matter, etc., for the specificpurpose of implanting a lateral unicondylar knee prosthesis is provided.This kit is designed to give an orthopedic surgeon the tools needed toperform the implant. The kit may include but is not limited to: (a) aset of femoral components; (b) a set of tibial components; (c) a set ofmobile bearing components; (d) a set of feeler gauges for determiningcorrect sizing of components; and (e) a set of instructions forperforming a lateral unicondylar knee implant surgery.

Each set is further broken down into specific components required forthe implant surgery, and may include the following:

(a) a set of femoral components including:

(i) at least one femoral drill guide;

(ii) at least one femoral prosthetic component;

(iii) at least one femoral prosthetic template component; and

(iv) a femoral instrument set.

(b) a set of tibial components including:

(i) at least one tibial prosthetic component;

(ii) at least one tibial prosthetic template component;

(iii) a tibial impactor; and

(iv) a tibial instrument set.

(c) a set of bearing components including:

(i) at least one mobile bearing member;

(ii) at least one mobile bearing template member; and

(iii) a bearing insert/removal tool.

(d) a set of feeler gauges for determining correct sizing of components;and(e) a set of instructions for performing a lateral unicondylar kneeimplant surgery.

As surgery is carried out under sterile conditions all of the equipmentis placed into a domed lateral sterilisation case to allow for easysterilisation of all components required in the kit.

Finally with reference to FIGS. 12 and 13 the method of implanting alateral unicondylar knee prosthesis 210 in a lateral articulatingportion of a femur 15 having a lateral condyle 60 for engagement with atibia 16 is described. Firstly, in flexion, the lateral collateralligament (LCL) is slack whereas the medial collateral ligament (MCL) istight. Therefore, it is not possible to determine the position of thelateral femoral condyle 60 from ligament balance, as is done inoperations to replace the medial compartment (discussed in more detaillatter). Therefore with the knee bent to approximately 90° flexion 220,an incision 230 is made through the skin, muscle and other soft tissue.

The method is now split into two distinct and different phases (resecttibia) 240 and (resect lateral femoral condyle) 250. The end portion ofthe tibia is resected 240 by firstly splitting the patella tendoncentrally with a knife. A vertical saw cut is then made through thepatella tendon with the knee flexed to 90°. The saw blade should touchthe medial side of the lateral condyle 60. The two landmarks, the centreof the tendon and the medial side of the lateral condyle 60 define theposition of the cut and ensure it is adequately internally rotated. Thesaw cut will usually be at the correct depth when its upper edge islevel with the bone surface.

A horizontal saw cut is then made, where the aim is for the horizontalcut to be approximately 8 mm below the original tibial articularsurface. If the site of the original surface can not be estimated, thecut should be approximately 2 or 3 mm below the eburnated bone. A tibialtemplate 41 is then inserted 241 and with the knee 1 fully extended andan assessment 242 is made to confirm that enough bone has been resectedfrom the tibia 16.

The next step 250 is to resect the lateral femoral condyle 60 to allowfor the insertion of the femoral component 30. The aim of femoralpreparation is to place the femoral component 30 as close to theanatomical position 290 as possible. The size of the lateral femoralcomponent 30 is selected using the same principles as is the medialcomponent and the lateral femoral condyle 60 is resected as describedabove with reference to FIGS. 8 to 11.

Trial components 38, 41 are inserted 300 and the extension gap ismeasured with the knee fully extended. A trial reduction is undertakento determine the appropriate mobile bearing 50 thickness and to ensurethere are no problems with the reconstruction. An appropriate mobilebearing 50 is one that is not tight in full extension. The tibialcomponent 40 as described in FIG. 4 above has a projection or keel 14for which further bone needs to be resected 243 from the tibial plateauto accommodate the keel 14 of the tibial component 40.

A final assessment 310 is then performed to ensure all gaps areacceptable and that there is free movement of the joint. The tibialprosthetic component 40 is then attached 260 to the resected portion ofthe tibia and the lateral femoral condylar component 40 are alsoattached 270 to the resected portion of the lateral condyle. Bothcomponents 30, 40 are then cemented 320 to their respective bones.Finally a mobile bearing 50 is inserted 280.

Although the present invention has been illustrated and described withrespect to exemplary embodiment thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omission and additions may be made therein and thereto, withoutdeparting from the scope of the invention. Therefore the presentinvention should not be understood as limited to the specific embodimentset out above but to include all possible embodiments that can beembodied within a scope encompassed and equivalent thereof with respectto the features set out in the appended claims.

1. A method of implanting a lateral unicondylar knee prosthesis in a lateral articulating portion of a femur having a lateral condyle for engagement with a tibia, the method comprising the steps of: bending a patient's knee such that the knee is not in full extension; making an incision through the skin, muscle, and other soft tissue until the damaged bone surfaces are exposed; resecting an end portion of the lateral tibia; resecting an end portion of the lateral femoral condyle; attaching a tibial prosthetic component to the resected end portion of the tibia; attaching a lateral femoral condyle prosthetic component to the resected end portion of the lateral femoral condyle; determining the thickness of a mobile bearing member with the knee in full extension; and inserting the mobile bearing member between the tibial prosthetic component and the femoral prosthetic component.
 2. The method according to claim 1, wherein the position of the lateral femoral condyle is determined anatomically with reference to the femur.
 3. The method according to claim 1, wherein the step of resecting the end portion of the tibia comprises: sawing vertically a cut through the centre of a patella tendon positioned in line with the medial side of the lateral femoral condyle; and sawing horizontally a cut that removes bone to accommodate the tibial prosthetic component and the mobile bearing member with the knee in full extension.
 4. The method according to claim 3, wherein sawing horizontally further comprises removing bone to approximately 8 mm below the original tibial articular surface or 2 to 3 mm below the eburnated bone of the tibia.
 5. The method according to claim 1, wherein the tibial prosthetic component comprises a smooth articular surface and a bone contacting surface opposite the articular surface.
 6. The method according to claim 5, wherein the smooth articular surface is formed in a convex shape.
 7. The method according to claim 5, wherein the bone contacting surface opposite the articular surface further comprises a projection extending from the bone contacting surface.
 8. The method according to claim 7, wherein the step of attaching the tibial prosthetic component comprises: resecting the bone to accommodate the projection extending from the bone contacting surface; and cementing the tibial prosthetic component to the tibia.
 9. The method according to claim 7, wherein the step of attaching the tibial prosthetic component comprises: resecting the bone to accommodate the projection extending from the bone contacting surface; and attaching a cementless tibial prosthetic component to the tibia.
 10. The method according to claim 1, wherein the step of resecting the end portion of the lateral femoral condyle comprises: drilling a hole in a notch between the lateral femoral condyle and the medial femoral condyle to accommodate an intramedullary rod; inserting a lateral femoral drill guide; adjusting the leg and the lateral femoral drill guide to be parallel to the intramedullary rod; drilling two holes through the lateral femoral drill guide; attaching a posterior saw guide to the lateral femoral compartment; and milling the bone from the lateral femoral condyle in both anterior and posterior directions.
 11. The method according to claim 1, wherein the femoral prosthetic component comprises a smooth articular surface and a bone contacting surface opposite the articular surface.
 12. The method according to claim 10, wherein the smooth articular surface is formed in a convex shape.
 13. The method according to claim 10, wherein the bone contacting surface opposite the articular surface further comprises a spigot extending from the bone contacting surface.
 14. The method according to claim 11, wherein the step of attaching the lateral femoral prosthetic component comprises: cementing the lateral femoral prosthetic component to the femur.
 15. The method according to claim 11, wherein the step of attaching the lateral femoral prosthetic component comprises: attaching a cementless lateral femoral prosthetic component to the femur.
 16. The method according to claim 1 wherein the mobile bearing member is a biconcave bearing having a first bearing surface which articulates with the smooth articular surface of the tibial prosthetic component and a second bearing surface which articulates with the smooth articular surface of the femoral prosthetic component.
 17. The method according to claim 1, wherein the tibial prosthetic component, the femoral prosthetic component and the mobile bearing member are made of a material selected from the group of titanium, titanium alloy, cobalt chrome alloy, ceramic, biocompatible composite, polymer, niobium, and steel alloy.
 18. A method of implanting a lateral unicondylar knee prosthesis in a lateral articulating portion of a femur having a lateral condyle for engagement with a tibia, the method comprising the steps of: bending a patient's knee such that the knee is not in full extension; making an incision through the skin, muscle, and other soft tissue until the damaged bone surfaces are exposed; resecting an end portion of the lateral tibia; sizing a tibial template component by fully extending the knee; resecting an end portion of the lateral femoral condyle; sizing a lateral femoral template component by fully extending the knee; and sizing a mobile bearing template member for insertion between the tibial template component and the lateral femoral template component. 